What else can boost iterative deepening with alpha-beta pruning?
First thing you're going to want to add is probably a Transposition Table, as also suggested by SmallChess.

Afterwards, I'd look into Aspiration Search and/or Principal Variation Search (also see this page).

Then I'd look into things like the Killer Move Heuristic, and maybe also see if you can simply implement existing parts of your engine more efficiently (e.g. use bitboards for your state representation).

Other than all of that, the chess programming wiki probably has lots of other interesting pages as well.